N7 CTF CRYPTO

First challenge

in this crypto challenge we get two files message.py nad output.py

analyse the source code

this is the encrypt function:

`def encrypt(message, key):
    encrypted_message = ""
    key_index = 0
    for char in message:
        key_value = int(key[key_index])
        encrypted_char = chr(ord(char) + key_value)
        encrypted_message += encrypted_char
        key_index = (key_index + 1) % len(key)
    return encrypted_message `

and this key funct:

`def generate_key(length):
key = ""
for _ in range(length):
    key += str(random.randint(0, 1))
return key`

get the flag

so the encryption adds 0 or 1 to the ord(message_char)

if 0 is added the character is not changed

so we can try decrypting the encrypted message using 2 keys the first one all zeros second one all ones

flag = 'itsfiuifs{esoprtfurbhpo' key = '11111111111111111111111' lenght = len(flag) for char in flag: message = "" key_index = 0 key_value = int(key[key_index]) encrypted_char = chr(ord(char) - key_value) message += encrypted_char key_index = (key_index + 1) % len(key) print(message)

Decrypted (Key 11111111111111111111111):

hsrehtherzdrnoqsetqagon

Decrypted (Key 00000000000000000000000):

itsfiuifs{esoprtfurbhpo

since they give us english plaintext we can just try to get the correct flag

itsfiuifs{esoprtfurbhpo

hsrehtherzdrnoqsetqagon

we get in result:

itseitherzeroortetargon

key

for the key we can just compare betwen the flag and the ecnrypted text

itsfiuifs{esoprtfurbhpo hsrehtherzdrnoqsetqagon

we get the key:

00010111110101001101111

Second challenge

this challenge is too easy just use https://www.dcode.fr/rsa-cipher

c = 2639222037939602930779997597686066660027731122115841970751307118936967286551172831245185216904150318662068646683922890543361488253707451777944539953999880820333353409477004444838728567823330779562645224253848830988777624421

e = 3

n = 105685955572725005347272960743239621411993077386863604096770961745412000510196377797206820581777171869540729112886024112689923030604026133879334659352568724562364148963053997247996020334986347187697178849374917417289646475949679535200176152455407176055260974820531493339297291633199170870295476113587842624831

we get the flag

flag:

N7-CTF{just_s1mple_m3th_765622}